WO2004055959A1 - Double-fed asynchronous machine without a slip ring - Google Patents

Abstract

The invention relates to a double-fed asynchronous machine that is devoid of a slip ring. Said machine comprises a main machine (4) with three-phase windings in the main rotor (32) and main stator (34) and an inducer machine (6) with a direct current winding in the inducer stator (34) and three-phase winding in the inducer rotor (36). The three-phase windings in the main rotor (32) and inducer rotor are mounted on the same shaft (8) so as to be mechanically stationary and interact with said shaft to give a rotating part. A converter (28) for converting a current having an output frequency produced in the stator winding of the inducer machine to a current having a desired frequency fed to the stator winding of the main machine is stationarily mounted on the shaft (8).

Description

description

Double-fed asynchronous machine without slip ring

The invention relates to a slip-fed double-fed asynchronous machine.

Double-fed asynchronous machines generally contain two three-phase windings, a stator winding fixed to the machine frame or housing and a rotor winding mechanically fixed to a shaft, the shaft being rotatably mounted with the rotor winding with respect to the stator winding. The rotor winding requires electrical rotor connections that are stationary, e.g. are accessible on the machine frame. The rotor connections are usually electrically connected to the rotor winding via slip rings which are mechanically fixed to the shaft in conjunction with carbon brushes which are resiliently attached to the machine frame and which slide on the slip rings and thus establish an electrical contact.

Asynchronous machines are used as motors or generators. Double-fed asynchronous generators are mostly used to generate electrical energy in wind turbines of the upper power class, ie from 1 MW upwards. Such

The arrangement is, for example, from “Warnecke, Otto; Siemens Energietechnik 5 (1983), issue β, p. 364ff ". Conventional double-fed asynchronous generators have slip ring rotors and three-phase windings in the rotor and stator. In wind energy plants, a variable speed of the generator is advantageous for various reasons Primary energy in the form of a gust of wind is first converted into an increase in the number of revolutions of the generator and only gradually the torque of the system is increased and thus the power output is increased. smoothed performance curve compared to a fixed speed system. Nevertheless, the generator at the output terminals should generate power with a grid-synchronous constant frequency. That is why the three-phase winding is used in the rotor to impress a regulated differential frequency there. The stator frequency results from the addition of the current speed of the generator and the difference frequency in the rotor. The difference frequency is selected, for example, in such a way that mains frequency is set at the output of the generator. Normally, the generator operates in a speed range that deviates upwards and downwards from its synchronous speed by about 10 - 30%. The difference frequency impressed in the rotor can be positive or negative. The direction of energy is therefore also positive or negative, which means that at sub-synchronous speeds, energy is fed into the rotor and the respective demand is dissipated to the standard current; at oversynchronous speeds, energy is drawn from the rotor and the respective rotor current generated is fed to the stator current or network. The control or stamping of the rotor frequency takes place via stationary, stationary converters, which are combined

Rectifiers and inverters. The rotor winding is usually connected to the converter in three phases via slip rings.

Slip ^rings' and carbon brushes as mechanical components are subject to considerable wear and cause, for example, coal dust as abrasion of the carbon brushes. As a result, a machine equipped in this way requires a lot of maintenance. Depending on the operating conditions, slip rings and brushes can lead to unplanned failures and thus have a negative impact on the availability of the machine.

For large wind turbines and especially in offshore applications, totally encapsulated generators are required for protection against environmental influences, in which the brush dust produced from electrically conductive graphite remains in the machine or in the machine nacelle and an additional source of error (e.g. winding short circuits) represents. To avoid the problem, a slip ring-free asynchronous machine is known from DE 101 41 486 I, for example, which fulfills the above-mentioned requirements. The disadvantage here, however, is that, starting from the converter, disturbing harmonics are fed into the power network connected to the machine. If the known asynchronous machine is operated as a generator, it is possible to control the electrical properties of the machine in the frequency converter. In this case, two induction machines connected in series are connected electrically. In order to be able to influence the electrical properties at the output of the second machine, the behavior of both machines must be taken into account in the controller. For example, the speed of the shaft is squared in the relationship between the output frequency of the converter and the output frequency of the generator. A regulation is therefore not complex.

The task now is to specify an asynchronous machine which is designed without a slip ring, has favorable harmonic properties and has control properties similar to a slip ring machine.

The object is achieved according to the invention by a slip-fed double-fed asynchronous machine with the features of claim 1. Such a machine contains a main machine with three-phase windings in the main rotor and main stand and an exciter with direct current winding in the exciter stand and three-phase winding in the exciter rotor. The three-phase windings in the main rotor and exciter rotor are mechanically fixed on the same shaft and together with it form a rotating part of the asynchronous machine. The asynchronous machine also contains a converter that is firmly attached to the shaft. The converter serves to convert a power generated in the rotor winding of the excitation machine, which has an output frequency determined by the speed of the shaft, into one of the rotor Power supplied to the main machine, which has a nominal frequency generated by the converter.

By placing the converter on the shaft, it is no longer connected directly to the power supply, both on the input and output side, but via one induction machine each. The transmission of harmonics into the power grid is largely suppressed because an induction machine has a much "softer" voltage curve shape than a converter. A control / regulation of the electrical properties of the machine in generator operation that may have to be carried out, as with slip ring machines, therefore only has to do this The behavior of a single induction machine, namely the main machine, must be taken into account, and the control can therefore be carried out relatively simply, as in a conventional double-fed asynchronous machine with slip rings.

The brushless and slip ring-free design of the asynchronous machine avoids abrasion of the brushes in the form of brush dust (graphite). The machine can no longer be contaminated by brush dust and graphite deposits are ruled out as a source of errors for winding short circuits. Brushes and slip rings are no longer necessary and therefore the maintenance work to be carried out on them. In addition, the risk of unplanned failures is reduced by the elimination of maintenance-intensive and error-prone parts, such as carbon brushes. The availability of the machine is increased. The task of the slip rings and carbon brushes, namely to transmit electrical power between the rotor and the stator, is taken over by the exciter by transmitting this energy according to the transformer principle by means of electromagnetic fields.

In a preferred embodiment, the target frequency of the power supplied to the main rotor can be specified. As a result, the asynchronous machine is particularly useful in generator operation Given the speed of the shaft, the frequency of the power generated on the main stand can be controlled, or it is made possible that the frequency of the power output on the main stand is kept constant when the speed of the shaft changes by controlling the target frequency.

In a further embodiment of the invention, a transmission device is provided which allows information to be transmitted without contact between the stationary and rotating part of the asynchronous machine. This information can e.g. Control signals that are transmitted from the stationary part of the machine to the (rotating) converter, e.g. the value of the nominal frequency of the converter can be determined, calculated or specified in the stationary part of the machine and then transmitted to the converter without contact. Information from the rotating part, e.g. the temperatures of the windings of the main and field rotors are transferred to the stationary part of the machine and processed there, for example. Such a transmission device for information transmission must contain at least two interacting reciprocals, one working as a transmitter and one as a receiver, and a transmission path between them. Since each other side works both as a transmitter and a receiver, these are referred to as transceivers. The attachment of the transmitting and receiving devices in the area of the machine shaft, in particular on the end face of the free end, i.e. that of a connected coupling, a transmission or the like, appears to be particularly favorable. facing end of the shaft. As a result, the air gaps to be bridged can be kept as small and constant as possible.

The transmission device for the contactless transmission of information can be a radio transmission link, in particular a radio LAN connection. A wireless LAN has the advantage over an infrared link that it is less sensitive to dirt. In one embodiment of the invention, the converter contains diodes which rectify the power generated in the field rotor winding of an output frequency. Furthermore, the converter contains an inverter for the subsequent alternation of the power into an alternating power of a set frequency. The converter also contains a DC circuit between the diodes and the inverter. An advantage here is that energy can easily be taken from the DC circuit to supply other electrical consumers.

This can e.g. the converter itself, in particular its inverter, or the transceiver of the transmission device attached to the shaft. The converter requires additional auxiliary energy to convert the power from an output frequency to a set frequency. In one embodiment of the invention, the converter draws the auxiliary energy from the DC circuit between the diodes and the inverter. Additional measures to supply the inverter with energy are thus avoided.

In another embodiment, the converter obtains the required auxiliary energy from an auxiliary winding attached to the shaft. The advantage here is that there is no intervention in the electrical circuit consisting of the exciter, converter and main machine by removing a variable amount of energy. Harmonics can thus be avoided better.

The current strength in the DC winding in the excitation stand can be made controllable. In this way, the power can be controlled, which is induced in the rotor winding of the excitation machine and which is fed to the converter or the three-phase winding of the main rotor, which in turn leads to control of the power delivered to the main stand in generator operation parallel to the network. In addition, the power factor can be controlled in this way by means of a control superordinate to the excitation power and setpoint frequency control in parallel operation with a stronger network (cos-φ control). In one embodiment of the asynchronous machine as a motor, it is designed for permanent oversynchronous operation.

In another embodiment, the asynchronous machine is designed as a generator, in particular as a generator in wind turbines. A wind turbine can be designed by using a corresponding gear between the axis connected to the rotor blades and the generator in such a way that even at the maximum achievable speed of the rotor blades, the speed of the generator remains so low that it always runs in sub-synchronous operation. As described above, power is only ever supplied to the main rotor. In this embodiment, the generator is therefore designed for permanent underfrequency operation.

In preferred embodiments, the generator is equipped with controllers that control the excitation power, e.g. For a given voltage, determine the current in the excitation machine and the target frequency for the converter. The regulators can be designed in such a way that a power with constant frequency, in particular the mains frequency, is always output at the output terminals of the three-phase winding in the main stand. This means that the generator can be operated to generate electricity on a power network with a fixed network frequency, e.g. the public electricity grid.

In a further preferred embodiment of the invention, a device is provided which allows the phase position between the current emitted by the generator and the voltage to be controlled. This is preferably done by a control system that is superior to the excitation power and setpoint frequency control. It is a so-called reactive power or cos-φ control. The ratio between active and reactive power can therefore be adapted to the network requirements.

In a particularly preferred embodiment when using the asynchronous machine as a generator in a wind turbine In order to optimize the efficiency of the wind turbine, joint control of the setpoint for the blade position of the wind turbine, the setpoint for the power supplied to the stator of the excitation machine and the setpoint frequency in the main run development are provided.

For a further explanation of the invention, reference is made to the exemplary embodiments of the drawing. Show it:

Fig. 1 shows an asynchronous machine according to the invention in the

Design as a generator with connection to a power supply network,

2 shows an asynchronous machine according to the invention in the design as a generator,

each in a schematic diagram.

1 shows an asynchronous machine 2 in operation as a generator, which contains a main machine 4 and an excitation machine 6. The asynchronous machine 2 is driven by a shaft 8, for example, by a windmill, not shown. The main machine 4 is connected via electrical lines 10 and 12 to a power grid 14, into which it feeds electrical energy. An excitation device 18 is also connected to the electrical network 14 via electrical lines 12 and 16 and feeds the excitation machine 6 with energy via an electrical line 20. The excitation device 18 contains a transformer 22, a rectifier 24 and a controller 26. The controller 26 regulates the voltage at the output of the main machine 4 by adjusting or regulating the excitation power, that is to say the direct power supplied to the excitation machine 6 (terminals 50; FIG. 2 ).

A converter 28 is electrically connected between the excitation machine 6 and the main machine 4, which converts the power of an output frequency generated or transmitted in the excitation machine converts a power of a target frequency and feeds it to the main machine 4. The output frequency is dependent on the speed of shaft 8 and, in the case of a wind turbine, therefore on the current supply of primary energy (ie the wind strength).

In previous embodiments of double-fed asynchronous generators, inverters 28 were always arranged between the lines 16 and 20 instead of the excitation device 18 - this corresponds to a placement between network terminals and rotor slip rings. As a result, 29 harmonics were fed into the power network 14 in the direction of the arrow. By positioning the converter 28 on the shaft 8 and electrically between the main machine 4 and the excitation machine 6, the feeding of harmonics into the power grid 14 is largely avoided. The harmonics generated by the converter in each case pass through an induction machine (4 or 6) before they can reach the power grid 14. However, induction machines have a very "soft" voltage curve shape, i.e. harmonics are very well damped.

In order to be able to operate the generator on a power grid 14 with a fixed grid frequency, the generator must always output power at this grid frequency. Since the speed of the generator can vary greatly, for example in wind turbines, frequency adjustment must take place. For this it is necessary that the set frequency can be specified in the converter 28. The setpoint frequency is determined in a controller 62 and transmitted to the converter 28 via the line 63. By regulating the power supplied to the exciter stand with the aid of the controller 26 and by selecting a suitable target frequency in the converter 28, the system is controlled in such a way that the line 10 from the asynchronous machine 2 always feeds power at the mains frequency into the mains 14, the generator thus works in synchronism with the network. In order to coordinate both controllers 26 and 62, a higher-level controller 64 is provided. With This can be used, for example, to implement a reactive power control for the power fed into the network 14. In parallel operation with a stronger network 14, the power factor cos-φ can thus also be regulated.

It can be seen in FIG. 2 that the main machine 4 of the asynchronous machine 2 contains the main stand 30 and the main rotor 32. The excitation machine 6 contains the excitation stand 34 and the excitation rotor 36. The main stand and rotor 30 and 32 and the excitation rotor 36 contain three-phase windings, the excitation stand 34 contains a direct current winding. Exciter rotor 36 and main rotor 32 are fixed mechanically on the shaft 8. A fan wheel 38, the converter 28 and the electrical lines 40 and 42 for connecting the converter 28 to the rotor windings 36 and 32 are also fixed mechanically on the shaft. The shaft is rotatable in bearings 44 relative to the housing 46 of the asynchronous machine 2 stored.

In operation, the axis 8 and the parts attached to it rotate in the asynchronous machine 2. The fan wheel 38 conveys cooling air through the asynchronous machine 2, e.g. to dissipate power losses in the form of heat from the machine in the windings or in the converter 28, ie to cool the machine. Attached to the housing 46 of the asynchronous machine 2 is a terminal box 48 which carries electrical terminals 50. On the one hand, the main stand winding 30 is connected to the terminals 50, e.g. to be able to dissipate the power generated in it in generator mode. On the other hand, the control 26 is connected to the terminals 50 in order to pass on a direct power fed in at the terminals 50 to the exciter stand winding 34 in a controlled manner.

On the end face 52 of the shaft 8, for example, the shaft of a wind turbine (not shown) engages the shaft 8 in a non-positive manner, so the shaft 8 is mechanically driven from the direction of the end face 52. On the opposite On the end face 54 of the shaft 8, a transmission device 56 is positioned for wireless information transmission, which contains a transceiver 58 which is mechanically fixed to the shaft and a transceiver 60 which is stationary with respect to the generator housing 46. A transmission path 61 is located between the two transceivers 58 and 60. With the aid of the transmission device 56 for information transmission and a line 63, for example, a setpoint can be transmitted without contact from a stationary setpoint generator in the controller 62 to the rotating converter 28.

In generator operation, the direct current winding of the excitation stand 34 has a direct current flowing through it, the output of which is regulated by the controller 26. The direct current in the exciter stand 34 generates a magnetic direct field in which the three-phase winding of the exciter rotor 36 rotates. This induces a voltage or power in the three-phase winding of the field rotor 36. The power thus generated in the exciter rotor 36 is fed to the converter 28 via the line 42. The alternating power is first rectified via diodes in the converter, for example into a DC voltage. From this DC voltage, all electrical consumers located on the shaft 8 can be supplied, for example the inverter in the converter or the transceiver 58 of the transmission device 56 for transmitting information, which is attached to the end face 54.

The rectified power is then converted by the inverter 28 into a power at the desired frequency and fed to the three-phase winding in the main rotor 32 via the feed line 40. The (rotating) three-phase winding in turn generates an alternating magnetic field which induces an alternating voltage of a total frequency in the three-phase winding of the main stator 30, the total frequency being dependent on the target frequency specified in the inverter 28 and the speed of the shaft 8. The size of the current in the The main rotor 32 is regulated by the magnitude of the current that is supplied to the exciter stand 34.

The target frequency, which is used in the inverter 28 as a reference variable for frequency clocking, that is to say the frequency in the main rotor 32, is formed in a stationary controller 62.

The nominal frequency is transmitted to the converter 28 without contact. The setpoint frequency acts via the inductive coupling between the field rotor 32 and the stator 30 on the nominal frequency of the power output by the generator at the terminals 50. In parallel operation with a stronger network 14, the output power of the generator 2 is thus also regulated.

According to the principle described above, only positive difference frequencies between the speed and the desired frequency can be generated, that is, power can only be supplied to the main rotor 32, since power transmission from the rotor 36 to the stator 34 of the exciter is not possible. The generator is therefore designed for permanent underfrequency operation. This is achieved through the gearbox tuning mentioned above. Since the power flow from the network 14 to the excitation device 18 takes place, which essentially feeds back hardly any harmonics and the power is supplied from the main machine 4 to the network 14 and hardly any harmonics are found here either, the generator according to the invention has a better voltage curve shape than conventional double-fed asynchronous generators.

A comparison of the controllers 26 and 62 with the blade adjustment and the power control of the wind turbine is possible and useful in order to optimize the efficiency of the wind turbine. The higher-level controller 64 is used for this purpose. The comparison of the controllers 26, 62 and 64 with one another or the combination of all controllers in a single control unit is also possible and useful in order to save costs and to implement a uniform control.

Claims

claims 1. Slip ring-less double-fed asynchronous machine (2) with a main machine (4) with three-phase windings in the main rotor (32) and main stand (30) and an excitation machine (6) with direct current winding in the exciter stand (34) and three-phase winding in the exciter rotor (36), whereby the three-phase windings in the main rotor (32) and exciter rotor (36) are mechanically fixed on the same shaft (8) and together with this form a rotating part, and with a converter (28) fixed on the shaft (8) for conversion a power of an output frequency generated in the winding of the exciter rotor (36) into a power of a target frequency supplied to the winding of the main rotor (32). 2. Asynchronous machine (2) according to claim 1, wherein the target frequency of the converter (28) can be predetermined. 3. Asynchronous machine (2) according to claim 1 or 2, with a transmission device (56) for contactless transmission of data from and to the rotating part, in particular for the transmission of a desired frequency to the converter (28). 4. Asynchronous machine (2) according to claim 3, wherein the transmission device (56) for contactless transmission Radio transmission link, in particular a radio LAN connection. 5. Asynchronous machine (2) according to one of the preceding claims, in which the converter (28) diodes for rectifying the power of an output frequency generated in the winding of the exciter rotor (36) and an inverter for reversing into an alternating power of the set frequency and one between Contains diodes and inverters. 6. Asynchronous machine (2) according to claim 5, in which auxiliary power is taken from the DC circuit, in particular for supplying power to the inverter and the transmission device (56). 7. Asynchronous machine (2) according to one of claims 1 to 5, in which the auxiliary energy required on the shaft (8), in particular by the inverter, is taken from an auxiliary winding attached to the shaft. 8. Asynchronous machine (2) according to one of the preceding claims, in which the current intensity can be controlled in the DC winding in the exciter stand (34). 9. Asynchronous machine (2) according to one of the preceding claims, which is designed as a motor. 10. asynchronous machine (2) according to claim 9, wherein the motor is designed for permanent oversynchronous operation. 11. Asynchronous machine (2) according to one of claims 1 to 8, which is designed as a generator, in particular as a generator in wind turbines. 12. Asynchronous machine (2) according to claim 11, wherein the generator is designed for permanent underfrequency operation. 13. Asynchronous machine (2) according to one of claims 11 to 12, with a controller (26) for regulating the excitation power. 14. Asynchronous machine (2) according to one of claims 11 to 13, with a controller (62) for regulating the target frequency. 15. Asynchronous machine (2) according to one of claims 11 to 14, with a controller (64) for regulating the power output on the main stand (30), such that this power is output at a constant frequency. 16. Asynchronous machine (2) according to one of claims 11 to 15, in which a cos-φ control of the power output from the winding of the main stand (30) is provided. 17. Asynchronous machine (2) according to one of claims 11 to 16, for use as a generator in a wind turbine, in which to optimize the efficiency of the wind turbine, a common control of the setpoint for the blade position of the wind turbine, the setpoint for the exciter stand (34) supplied power and the value of the target frequency is provided in the main rotor (32).